The present invention relates to a furnace protective device for a condensing furnace to terminate the operation of a burner when there is an abnormal accumulation of condensate in the secondary heat exchanger. Particularly, the invention relates to a burner protective device for a heating system having a primary heat exchanger and a secondary heat exchanger connected in series. A burner burns a gas-air mixture and the resulting products of combustion flow through the primary heat exchanger and then through the secondary heat exchanger.
The combustion products generally remain gaseous in the primary heat exchanger. However, the cold room air contacting the walls of the secondary heat exchanger produce condensation of the combustion products, releasing latent heat of vaporization that transfers additional heat to the room air. This condensate flows with the remaining combustion gases from the secondary heat exchanger tubes to an outlet manifold. The gases escape through a flue. See, for example, U.S. Pat. Nos. 4,738,307 and 4,729,328.
The condensate normally flows from a secondary heat exchanger through an outlet that is piped to a suitable drain. If condensate backs up in the secondary heat exchanger for any reason, such as from a blockage of the condensate drain, the liquid may build up in the secondary heat exchanger to a level that will reduce combustion air flow and affect the combustion quality to the point that carbon monoxide will be produced.
The present invention provides a protection device to turn off the gas to the burner when the level of the condensate in the secondary heat exchanger rises to an abnormal level. This protective device is connected to an ignition control module. The control module includes a D.C. circuit that is closed by the presence of a flame at the burner and that opens when the flame is extinguished. A relay circuit to a gas valve is deactivated by the ignition control module to close the gas valve when extinguishment of the flame opens the circuit through the burner.
In the conventional mechanism, a pressure switch is connected in the thermostat circuit in series with the ignition control module. The pressure switch is positioned to respond to (negative) pressure in the condenser outlet manifold produced by an induction blower. If the condensate level rises to an abnormal level, the pressure switch will open the thermostat circuit to the ignition control module, shutting down the system, which closes the gas valve.
This conventional mechanism has several drawbacks. Primarily, the pressure switch is too sensitive and sometimes reads pressure signals falsely and shuts down the system. Different sized furnaces require different settings of the pressure switch. It is relatively costly, both in materials and installation. Overall, it is relatively expensive.